Conventional cascade impactors have been used extensively for fractionating airborne particles according to their aerodynamic sizes, enabling the size distribution to be determined by analyzing the collected particles. However, this technique has several inherent problems, including stage overloading and particle reentrainment, and introduces errors in the measured aerodynamic size distribution. In addition, aerodynamically separated aerosols cannot be used directly as a source of monodisperse aerosols, because the particles have been collected on the impactor stages. It is also not possible to take either the coarse or fine particle output of one stage and use it as the input of a second stage for obtaining increased narrow sized fraction of the separated particle stream.
These limitations are avoided in the dichotomous virtual impactor. This device uses the same principle of inertial separation, but the impaction plate is replaced by a region of relatively stagnant air contained in the cavity of a receiving probe. A virtual surface is formed by deflected streamlines that are similar to those in conventional solid plate impactors. The fine particles follow the streamlines of the major air flow, while the coarse particles, with greater inertia, pass into the forward minor flow region. Both size fractions can, subsequently, be ducted for any desired methods of analysis or collections, including direct-reading, continuous instrumentation, or can be sent to a subsequent stage or stages for further refinement.
Because a virtual impactor does not collect particles, but merely redirects them into two different air streams according to the cutoff characteristic, it is generally free from problems of particle bounce and reentrainment that often occurs in other inertial, size separating devices. However, in drawing minor flow through the receiving or collection probe, the coarse particle flow entrains a small amount of fine particles. This contamination is an intrinsic disadvantage of virtual impactors of conventional design.
Masuda, et al, in the publication "An Improved Virtual Impactor for Particle Classification and Generation of Test Aerosols With Narrow Size Distributions", J. Aerosol Sci Vol. 10 pp 275-287 (1979) described a virtual impactor that reduced contamination by confining the input aerosol flow between an inner and an outer flow of particle-free air. However, under certain operating conditions, apparent distortions in the annular flow of the aerosol may exist, adversely affecting separation efficiency. More particularly, Masuda, et al. found that with their configuration, if the ratio of outer clean air flow rate to total flow rate exceeded 0.2, separation fell below predicted values. Masuda, et al. explained this behavior on the basis of distortion of the annular aerosol flow. This instability is a serious detriment to utilization of the Masuda, et al. structure.
As is the case with real impactors, the two parameters used to characterize the performance of a virtual impactor are separation efficiency and wall loss. A good virtual impactor should have a sharp separation curve with little wall loss and little fine particle contamination in the coarse particle fraction. While Masuda, et al. attempted to address the problem of separation efficiency, other strategies are needed to reduce the wall losses. Loo U.S. Pat. No. 4,301,002 describes a variety of dimensional relationships between the components of an acceleration nozzle and collection probe of a virtual impactor and such parameters are also described by Chen, et al. in the article "A Novel Virtual Impactor: Calibration and Use", J. Aerosol Sci. Vol. 16, No. 4, pp. 343-354 (1985). Achievement of an optimum design requires balancing between the movement of the various particle and gas streams and spacing and sizing of the confronting components to obtain both low wall loss and good separation efficiency.